Something Wicked This Way Comes.

Underdark

The Underdark is the vast, possibly larger and more expansive network of underground caverns and tunnels underneath the surface of the world. It was home to a host of evil beings driven deep into the caverns at the end of the age of demons.

World Above
The name those who live in the Underdark give the surface.
Upperdark
The first three miles below the surface. It iss here where the surface dwellers and those in the Underdark most often meet.
Middledark
Located three to ten miles below the surface, this layer iss where most of the Underdark cities are located.
Lowerdark
Ten miles or more below the surface, the Lowerdark is where even those who knew the Underdark loath to go.

It is not always possible to travel from one end of the Underdark to the other. The Underdark is divided into several “domains” that are similar to continents on the world above. While it was possible to travel from one place to another within a “domain”, separate domains tended to have very few passages linking them. The major domains of the Underdark were the Buried Realms, the Darklands, the Deep Wastes, the Earthroot, the Glimmersea, Great Bhaerynden, the Northdark and Old Shanatar. The Northdark is home to Menzoberranzan, the largest city in the Underdark and one of the largest in the world.

The Underdark’s underground landscape varies from small subterranean fissures almost impossible for adventurers to crawl through to wide expanses of deep water that hide coral caves, hollowed out sections of ice in which creatures live, and places where fungus, bones, or even pure force formed “caves” large enough to hold entire cities and rumor has it, entire countries.

The varied landscape is as dynamic as the surface. Some areas are ever-changing over the course of time by underground magma rivers or the emptying of underground lakes due to cracks in the rock formed by earthquakes. This means maps of the Underdark are highly unreliable.
The Underdark’s environment varies vastly from the surface world, making adventuring through it a dangerous expedition. Just like trekking through a desert, adventurers traversing the Underdark’s passages needed to be well prepared. The Underdark is mainly without any source of light save for the occasional luminous rocks or fungi. Large portions of it are also without food or water, making well-stocked food supplies a must-have when exploring uncharted terrain underground. Lack of light combined with a tomb-like silence make traveling something only for the strong-hearted. The nerve-wracking silence makes even experienced adventurers grow agitated after a few days. Just like deep dungeons, the air could grow stale and even poisonous in the Underdark. It could contain explosive gases from underground volcanic activity or other deadly gases bubbling out of deep fissures in the rock. The temperature underground varies according to depth and the proximity of thermic activities in the area. Throughout the year the temperature remains stable and moderate in most regions, usually slightly clammy or chilly. The water underground is extremely cold and poses a hypothermia risk. Plant life and animal life rival that on the surface. Ranging from small insects to hook horrors or shadow dragons which set up their realms in the down below, everything can be expected. Plant life was unable to utilize sunlight as a source for energy, and thus took on strange forms adapted to life underground.

In addition to those dangers, the Underdark was also the home of many predators and often-hostile races, including beholders, derro, drow, duergar, dwarves, illithids, kuo-toa and svirfneblin.

The Underdark is extremely dangerous, especially to those not native to it. There are the usual dangers associated with caverns: claustrophobia, occasional poor air circulation, and getting lost. There is no light except for occasional patches of fluorescent fungi; most Underdark inhabitants either have highly developed senses other than sight or have developed darkvision. Food could be extremely difficult to find, and much of the natural vegetation is poisonous.